A switch mode power converter (also referred to as a “power converter”) is a power supply or power processing circuit that converts an input voltage waveform into a specified output voltage waveform. Controllers associated with the power converters manage an operation thereof by controlling the conduction periods of switches employed therein. Generally, the controllers are coupled between an input and output of the power converter in a feedback loop configuration (also referred to as a “control loop” or “closed control loop”).
Typically, the controller measures an output characteristic (e.g., an output voltage) of the power converter and based thereon modifies a duty cycle of the switches of the power converter. The duty cycle is a ratio represented by a conduction period of a switch to a switching period thereof. Thus, if a switch conducts for half of the switching period, the duty cycle for the switch would be 0.5 (or 50 percent). Additionally, as the needs for systems such as a microprocessor powered by the power converter dynamically change (e.g., as a computational load on the microprocessor changes), the controller should be configured to dynamically increase or decrease the duty cycle of the switches therein to maintain the output characteristic at a desired value.
In combination with the controller, a driver is often employed to provide a drive signal to the switches of the power converter as a function of a signal from the controller. Assuming that the switches of the power converter are metal oxide semiconductor field effect transistors (“MOSFETs”), the driver is referred to as a gate driver and provides a gate drive signal to a gate terminal (i.e., a control terminal) of the MOSFET to control an operation thereof. Providing a gate drive signal with a limited control voltage range (or “gate voltage limit”) for a MOSFET is of particular interest in the design and implementation of power converters. In an exemplary application, the power converters have the capability to convert an unregulated input voltage such as five volts to a lower, regulated, output voltage such as 2.5 volts to power a load.
As discussed above, the power converters are frequently employed to power loads having tight regulation characteristics such as a microprocessor with, for instance, five volts provided from a source of electrical power (e.g., a voltage source). To provide the voltage conversion and regulation functions, the power converters include active switches such as the MOSFETs that are coupled to the voltage source and periodically switch a reactive circuit element such as an inductor to the voltage source at a switching frequency that may be on the order of five megahertz. To maintain high power conversion efficiency and low cost, the MOSFETs employed for the switches in the power converters are generally configured with fine line structures and thin gate oxides. The aforementioned structures that form the MOSFETs, however, present new design challenges associated with the control signals such as a gate voltage adapted to control the conduction periods of the switches.
For instance, recently designed MOSFETs for the power converters can reliably sustain control signals of about 2.5 volts from the gate terminal to the source terminal, whereas MOSFETs of earlier designs were able to sustain control signals of 20 volts or more. Additionally, the power converters often employ a P-channel MOSFET as a main switch therein. Inasmuch as the P-channel MOSFET is generally coupled to the input voltage (e.g., nominal five volts) of the power converter, the gate voltage is desirably controlled to a value of the input voltage (again, five volts) to transition the switch to a non-conducting state. Conversely, the P-channel MOSFET is enabled to conduct at a gate voltage equal to the input voltage of five volts minus 2.5 volts, which represents about the maximum sustainable voltage from the gate terminal to the source terminal of the switch (also referred to as a “gate-to-source voltage limit” or the “gate voltage limit” of the switch).
It is relatively common to employ a driver for a P-channel MOSFET that includes a series-coupled, totem-pole arrangement of a P-channel and N-channel MOSFET with coupled gate terminals. In the environment of a power converter, the totem pole driver (as the driver is customarily designated) for the P-channel MOSFET is coupled to the source of electrical power for the power converter and the controller of the power converter. The drive signal is generated from a junction coupling the drain terminals of the P-channel and N-channel MOSFETs of the totem pole driver. When a signal from the controller to the totem pole driver is high, the drive signal is essentially grounded. When the signal from the controller to the totem pole driver is low, the drive signal is substantially equal to the input voltage of the power converter. In effect, the drive signal from the totem pole driver exhibits voltages over the entire voltage range of the source of electrical power for the power converter. Alternatively, the driver may be described as providing a drive signal referenced to ground when its output is low, and a drive signal referenced to the input voltage when its output is high.
When providing a drive signal to a P-channel MOSFET (or any switch for that matter) having a gate voltage limit of 2.5 volts, and in the environment of a power converter having a nominal input voltage of five volts, the extended voltage range present on the gate terminal of the switch may break down the integrity of the thin gate oxide of the switch. In other words, when the input voltage to the power converter which is translated into the drive signal to the switch under certain conditions as described above exceeds the gate voltage limit thereof, the switch may be damaged and fail. Thus, the totem pole driver and other presently available drivers are typically not practical for applications wherein the switch to be driven exhibits a smaller gate voltage limit from the gate terminal to the source terminal thereof.
Another level of complexity arises when the switch to be driven and the driver are referenced to different voltages. In the environment of the power converter described above, circuitry that embodies the driver may be coupled to ground and referenced to a ground potential and the switch to be driven may be referenced to, for instance, the input voltage of the power converter. As a result, the driver is referenced to a ground potential and the switch is referenced to an input voltage such as an unregulated five volt input voltage. Thus, in conjunction with the control voltage limit, the driver should be adapted to drive a switch referenced to another voltage level as described above and adaptively perform the necessary voltage translation to the another voltage level to provide the drive signal.
Accordingly, what is needed in the art is a driver for the power converters, and a method of driving a switch thereof, that takes into account a control voltage limit associated with a switch (i.e., the gate voltage limit for a MOSFET) of the power converter referenced to a voltage level different from the driver that overcomes the deficiencies in the prior art.